The application of duplex printer which prints simultaneously on front and back sides of printing paper is becoming increasingly wider. A U.S. Pat. No. 6,784,906 discloses a duplex thermal printer, as shown in FIG. 1, which includes a first print head 150 and a first platen 170 which are supported on a first arm 130, and a second platen 180 opposite to the first print head 150 and a second print head 160 opposite to the first platen 170 which are supported on a second arm 140. Thermal paper 10 with thermal coating on both sides passes between the first print head 150 and the second platen 180, and between the second print head 160 and the first platen 170 along a paper conveying direction; the first print head 150 faces the front side 3 of the thermal paper 10 while the second print head 160 faces the back side 4 of the thermal paper 10; when both the first print head 150 and the second print head 160 generate heat under control respectively, preset images or characters are printed out on the front and back sides of the thermal paper 10.
Since the print head is an expensive component for constructing a printer, a duplex thermal printer is more expensive than a single-sided thermal printer. Therefore, on one hand, when the user, who needs single-sided printing only at present but probably needs duplex printing in the future, purchases a duplex thermal printer at the earlier stage, the initial cost of purchasing apparatus is increased and the utilization of the apparatus is not high at the earlier stage; on the other hand, when the user, who has purchased a single-sided thermal printer at the beginning, purchases a duplex thermal printer again when needing duplex printing, it not only needs to spend money on purchasing apparatus again, but also causes the wasting of apparatus because the single-sided printer already purchased is left unused.